Coatings of precisely ordered colloidal particles on solid surfaces are useful in many areas of science and technology. Randomly arranged colloidal particle coatings have been shown to be useful for interference and antireflection coatings (Iler, U.S. Pat. No. 3,485,658) and for tamper layers in fusion targets (Peiffer and Deckman, U.S. Pat. No. 4,404,255). Ordered arrays of colloidal particles coated on surfaces are useful either as a diffraction grating, an optical storage medium or an interference layer. Monolayer thick arrays of both random and ordered colloidal particles have been shown to be usable as a lithographic mask for the preparation of precisely controlled surface textures (Deckman and Dunsmuir, U.S. Pat. No. 4,407,695). Surface textures lithographically prepared from colloidal particle monolayers can contain uniformly sized microstructures over large areas, which are difficult to prepare with conventional lithographic techniques. Uses for uniformly sized, large area surface textures include selective solar absorbers, Craighead et al, U.S. Pat. No. 4,284,689, optical gratings and optically enhanced solar cells (Deckman et al, U.S. Pat. No. 4,497,974). The present invention relates to a method for preparing densely packed colloidal particle coatings which are free of defects.
The technology of coating a substrate with a particular type of monolayer thick random array of colloidal particles is well known. Such coatings will be called random colloidal coatings and methods for producing them are described by Iler in U.S. Pat. No. 3,485,658, as well as in Iler, Journal of Colloid and Interface Science 21, 569-594 (1966); Iler, Journal of the American Ceramic Society 47 (4), 194-198 (1964); Marshall and Kitchener, Journal of Colloid and Interface Science 22, 342-351 (1966); and Peiffer and Deckman, U.S. Pat. No. 4,315,958. These coating techniques deposit a random array of colloidal particles on the substrate utilizing an electrostatic attraction. When the colloidal particles are electrostatically attracted to a substrate they adhere at the point where they strike the surface. Electrostatic attraction occurs because a surface charge opposite to that of the substrate is induced on the colloidal particles. In this type of colloidal monolayer particles are randomly arranged spaces will exist between most of the particles. Examples of spaces between particles in random colloidal coatings are shown in FIGS. 1 and 2. FIG. 1 is an electron micrograph showing the ordering of monodisperse spherical latex particles in a random colloidal coating. Spaces between particles are clearly apparent in the micrograph. FIG. 2 is an electron micrograph showing the ordering of 2 .mu.m polystyrene latex particles in a random colloidal coating. The spaces between particles in random colloidal coatings arise from limitations on the number of particles that can diffuse to the surface to be coated and electrostatically adhere to form a monolayer. Random arrays are produced by immersing a substrate into a sol under conditions of Ph such that the surface of the substrate and the colloidal particles have charge of opposite sign. The colloidal particles diffuse through the sol to the substrate surface where the opposite charges interact to electrostatically bond the particle to the substrate. After the surface to be coated has achieved a given density of coverage of colloidal particles, which varies depending on the details of the coating process, the remaining uncoated surface is electrostatically screened by the presence of the adjacent adhered particles such that other particles diffusing to the surface are repelled back into the sol.
Formation of ordered colloidal particle arrays has been disclosed by Deckman and Dunsmuir, U.S. Pat. No. 4,407,695 (1983). In this process, ordered arrays of colloidal particles are formed by spin coating. Ordering of the particles occurs because the sol flows across the substrate at high shear rates while the excess coating material is being dispelled to produce densely packed microcrystalline arrays. The colloid must wet the substrate and spin speed must be optimized. If the spin speed is too low a multilayer coating will be produced and if the final spin speed is too high voids will occur in the monolayer coating. Other factors such as rheology of the sol, particulate concentration, substrate surface chemistry, and differential charge between substrate and colloid must be optimized for each particle size. A systematic method for optimizing these factors requires detailed understanding of the dynamics of the coating process which is not presently available. For spheres outside the 0.3-1.0 .mu.m size range, optimization of the coating process can be quite difficult. Imperfections in particulate ordering include point defects, dislocations, and grain boundaries. The largest number of submicron spheres observed in a single crystallite is 10.sup.5 and typical grains contain 50-1000 spheres. FIG. 3 is an electron micrograph showing the microcrystalline ordering of spin coated monodisperse polystyrene latex particles. FIG. 3 shows packing defects on part of a 3 in. silicon wafer which was uniformly coated with microcrystalline arrays of 0.497.+-.0.006 .mu.m spheres. The coating was prepared by flooding a surfactant cleaned wafer with polystyrene latex (Dow Diagnostics lot 1A27) containing 15 wt. % solids and spinning at 3400 rpm until dry.
The present invention describes a method for preparation of a third class of colloidal particle array with distinctly different properties from either ordered or random colloidal coatings. Most notable of these differences are control, the removal of empty spaces between particles that are found in random colloidal coatings, and the ability to produce either random or ordered coatings using a single coating technique.